The present invention relates to a method of producing an aromatic heterocyclic copolymer having excellent heat resistance, mechanical properties, chemical resistance, electric properties, etc., and to a method of producing a molecular composite material comprising such an aromatic heterocyclic copolymer and a matrix polymer suitable as a structural material for aircrafts, automobiles, spacecrafts, etc.
Recently, to achieve the reduction of weight of aircrafts, automobiles, etc., plastic materials called "engineering plastics" having excellent mechanical properties, heat resistance, etc. have been increasingly used. Apart from this, development has been actively carried out to provide composite materials such as FRPs consisting of plastic materials and high-strength, high-modulus fibers such as carbon fibers, and these composite materials are being used in many applications.
It is known that the strength of these composite materials largely depend not only on the strength of plastics and reinforcing fibers but also on the interfacial bonding strength of the fibers to the matrix resins. Also, the impregnating capability of matrix resins into the fiber-reinforced preforms affect the easiness of production of composite materials and the strength of the resulting products. Accordingly, the use of high-strength, high-modulus fibers and resins would not necessarily result in composite materials having excellent strength.
Under such circumstances, to overcome the above problems, proposals have been made to develop high-strength polymer blend composites (molecular composite materials) by finely dispersing rigid polymers such as aromatic polyamides, etc. in matrix resins to a molecular level.
Aromatic polymers suitable for molecular composite materials include those containing heterocyclic repeating units such as thiazole rings, imidazole rings, oxazole rings, etc. Among them, an aromatic polythiazole having a thiazole ring is highly promising because of its excellent mechanical strength.
In the meantime, the homogeneous dispersion of a reinforcing polymer such as an aromatic polythiazole in a matrix polymer cannot be achieved if the reinforcing polymer is simply blended with the matrix polymer, because of the rigidity of the reinforcing polymer which makes it less compatible with the matrix polymer. This means that simple blending fails to provide a molecular composite material having excellent mechanical properties. Therefore, various attempts have been made so far.
For instance, Japanese Patent Laid-Open No. 1-287167 discloses a method of producing a polymer composite comprising the steps of introducing a polymer solution mainly containing (A) a reinforcing polymer consisting of polythiazole having a substantially rod-shaped skeleton and (B) a matrix polymer fusable with the reinforcing polymer into a solidification bath, and forming it into a film, the above polymer solution showing an optical anisotropy and being solidified via a phase apparently having an optical isotropy, after immersion in the solidification bath.
Japanese Patent Publication No. 2-7976 discloses a polymer composition comprising a reinforcing polymer A consisting of polythiazole having a substantially rod-shaped skeleton, and a matrix polymer B consisting of a less-crystallizable aromatic copolyamide having a glass transition temperature of 200.degree. C. or higher and a flow-initiation temperature of 500.degree. C. or lower, a weight ratio of A/(A+B) being 0.15-0.70. When the aromatic copolyamide is kept at a temperature between its glass transition temperature and its flow-initiation temperature for an arbitrary period of time within 5 hours, the resulting crystals have apparent sizes of 25 .ANG. or less.
However, in the methods of producing polymer composites as disclosed in Japanese Patent Laid-Open No. 1-287167 and Japanese Patent Publication No. 2-7976, homogeneous dispersions of the reinforcing polymers in matrix polymers are not expected. This means that the resulting molecular composite materials do not show largely improved mechanical strength, etc. This appears to be due to the fact that the rigid reinforcing polymers and the matrix polymers do not show good compatibility with each other.
Thus, proposals have been made to provide a method of producing a molecular composite material of a rigid aromatic polymer without resorting to direct mixing of the rigid aromatic polymer and a matrix polymer, the method comprising the steps of homogeneously mixing a prepolymer of the rigid aromatic polymer and a matrix polymer or its prepolymer in an organic solvent, removing the organic solvent and then heating it so that the rigid aromatic polymer is formed from its prepolymer in the molecular composite material (Japanese Patent Laid-Open Nos. 64-1760 and 64-1761). By the above methods, molecular composite materials having relatively good mechanical strength, etc. can be produced.
However, the inventors have noticed that when thermal compression moldings of these molecular composite materials are produced from the aromatic polythiazole prepolymers and thermoplastic resins as matrix polymers by the methods of Japanese Patent Laid-Open Nos. 64-1760 and 64-1761, part of thiol groups in the prepolymers is subjected to a thiazole ring-closure reaction in the process of thermal forming of homogeneous mixtures of the aromatic polythiazole prepolymers and the matrix polymers, and that the resulting molecular composite materials show poor mechanical properties.
With respect to the rigid aromatic polymers, polymers having in their repeating units heterocyclic rings such as a thiazole ring, an imidazole ring, an oxazole ring, an oxazinone ring, etc. have been attracting much attention as high-strength, high-modulus, high-heat resistance polymers with high rigidity.
Among them, an aromatic polythiazole having a thiazole ring can be expected to show good properties as a plastic substitute for metal materials when used alone or in combination with other engineering plastics.
However, the rigid aromatic polymer such as the aromatic polythiazole generally has a poor solubility in organic solvents, etc. due to its high rigidity, and it is soluble only in limited kinds of strong acids such as metasulfonic acid, chlorosulfonic acid, etc. Also, it has only poor elongation and flexibility, meaning that it is poor in moldability. Accordingly, it is difficult to mold the rigid aromatic polymer without mixing with other materials.
Also, even in the case of forming a composite material of the rigid aromatic polymer and a matrix polymer, the above problem of poor moldability still remains. Further, since the rigid aromatic polymer does not show a good compatibility with the matrix polymer, it is difficult to blend the rigid aromatic polymer with the matrix polymer at a desired proportion.
To overcome such problems, it is possible to introduce a flexible moiety into the rigid aromatic polymer, thereby improving the moldability of the rigid aromatic polymer. Also, to improve the compatibility of the rigid aromatic polymer with the other polymer, it is possible to introduce a moiety compatible with the other polymer, which has the same structure as or a similar structure to the repeating unit of the other polymer, into the rigid aromatic polymer.
One example of such an attempt is disclosed by Japanese Patent Laid-Open No. 63-256622, which is a method of producing an aromatic heterocyclic block copolymer by reacting a particular aromatic oligomer with a monomer having a benzene ring provided with at least one carboxyl group and at least one amino group in a polyphosphoric acid. The aromatic heterocyclic block copolymers obtained by this method have rigid molecular chain moieties and soft chain moieties.
However, in the above method, the synthesis of the aromatic heterocyclic block copolymer is conducted while being heated in a polyphosphoric acid. Also, when the aromatic heterocyclic block copolymer obtained by this method is combined as a reinforcing material with another polymer to provide a molecular composite material, the aromatic heterocyclic block copolymer does not show a high compatibility with the matrix polymer because it already has a rigid molecular chain moiety such as a thiazole ring. Therefore, the aromatic heterocyclic block copolymer obtained by the method of Japanese Patent Laid-Open No. 63-256622 is not suitable for use in a molecular composite material.